It is clear (from our current processes, past practice, etc.) that the IESG can add a new AD seat at will.
I am not sure how/if we can create a new position called "Assistant IETF Chair"... Of course, there isn't anything stopping you, Harald, from delegating IETF chair responsibilities to anyone you like, so we could just create another General AD position and you could delegate to that person.
Is there a reason why you think it is important or desireable to call this person "Assistant IETF Chair" instead of just having them be a General AD?